1. Field of the Invention
The present invention relates to a light emitting device and to a method for manufacturing a light emitting device. Specifically, the present invention relates to a light emitting device and a method for providing a light emitting device based on the principle of photon up-conversion.
2. Related Technology
In a number of systems, it has been observed that irradiation by light with longer wavelength causes emission of a light with shorter wavelength. This phenomenon, which is called “photon up-conversion” or “up-conversion” is mostly associated with high light intensities available from coherent light sources, such as lasers. The already existing examples of up-conversion, i.e. the generation of photons, notably spectrally blue-shifted to the wavelength of the excitation photons by simultaneous or sequential absorption of two or more photons with lower energy has always been associated with the use of coherent light sources and are not adapted to be used with non-coherent light sources. Another limitation of theses existing methods are the required high light intensities, with power densities commonly on the order of MW/cm2 and GW/cm2, which cannot be reached by means of incoherent light sources.
In the state of the art several approaches have been made in order to adopt the physical principle of up-conversion also for incoherent light sources. Document Baluschev, S. et al.: “Up-conversion with ultra broad excitation band: simultaneous use of two sensitizers” Applied Physics Letters 90, 181103 (2007) provides a composition providing a photon up-conversion based on triplet-triplet annihilation in a bi-molecular system. This document provides a possibility for up-converting the red part of the sun spectrum realised by ultra low excitation intensity and ultra broad excitation spectrum. Since a triplet-triplet annihilation supported by molecular up-conversion process is used this process is independent on the coherence of the excitation light. The optical excitation of the system occurs by resonant single photon absorption: consequently, the efficiency of the up-conversion process depends on the material properties but does not in any way depend on the coherence of the photons used for excitation.
In document EP 1 484 379 A1 a system for up-converting photon energy is provided. Here for a composition for photon energy up-conversion comprising at least two components is provided, wherein a first component is capable of absorbing energy at a first wavelength region and wherein a second component is capable of emitting energy at a second wavelength region, wherein the second wavelength region is smaller than the first wavelength region and wherein, upon absorption of energy by the first component the second component emits energy at said second wavelength region. Hereby as components two organic, respectively organometallic compounds are used. One of the two components acts as a sensitizer absorbing energy of a first, longer wavelength and storing this energy for a short time (typically microseconds) in a metastable triplet state. Compounds like metalloporphyrins are typically used for this purpose.
One disadvantage of the prior art is that the system was optimized for solar cell applications and has a limited applicability for light-emitting diodes due to the emission of only one single wavelength by the up-conversion composition (many applications of light-emitting diodes, especially in the promising fields of general lighting, require white LEDs with a broad emission spectrum over a wide range of emission wavelength). On the other hand, when providing additional compounds within the composition, in order to achieve emission of further wavelengths, this makes the structure of the composition complicated and the composition difficult to manufacture and complicated.